The slotted core cable developed more than 30 years ago distinguishes itself especially for its high tensile and compression resistance and its compact construction, in spite of the large number of the optical fibers arranged in the slots of the central element. Optical cables of this kind are f.e. described in U.S. Pat. Nos. 5,517,591 and 5,199,094.
An essential component of the slotted core cable is the cylindrical central element, on whose jacket several slots are located, each of them open to the outside, in the form of a helix or spiral, if need be with periodically changing rotation direction. The process for the manufacture of such a central element can be found in U.S. Pat. Nos. 4,997,258 and 5,380,472.
The invention concerns a cable containing an optical transmission element with a central element and optical fiber ribbons arranged in the slots of the central element. The invention also concerns a process for the manufacture of such a cable.
In order to increase the number of the optical fibers (LWL) serving as optical transmission elements, consisting of a glass core (refractive index nx), a glass jacket (refractive index nm<nx) and a single or multi-layer protective covering (coating) in the slotted core cable, typically 8-16 optical fibers (LWL) are mechanically combined into a ribbon, and several of these ribbons are inserted into the slot of the central element one above the other in the form of a stack. U.S. Pat. Nos. 4,997,255 and 5,380,472 are especially relevant here. If the slot in the outer area of the central element describes a helix, whose rotation direction changes periodically, the optical fiber ribbons are twisted respectively, and thus subjected to a so-called SZ-stranding. The torsion thus produced in the optical fiber ribbons induces elastic forces, which cause the optical fiber ribbons in the slot to assume a preferred direction. Due to this alignment of the optical fiber ribbons in the slot the cable has two developed main axes with different bending behavior. This results in the following disadvantages:                a) The lengths of the individual optical fiber ribbons are not equally distributed onto the areas subjected to strain during bending of the cable. Especially the outer optical fiber ribbons of the stack are subject to high mechanical stresses, so that their signal attenuation is significantly increased due to micro and macro-bending.        b) The preferred alignment of the optical fiber ribbons leads to a mechanically unstable configuration at small bending radii. During bending of the cable, this can lead to spontaneous change in the order of the optical fiber ribbons in the cable. This also increases attenuation.        